Stress direction history of the western United States and Mexico since 85 Ma

[1] A data set of 369 paleostress direction indicators (sets of dikes, veins, or fault slip vectors) is collected from previous compilations and the geologic literature. Like contemporary data, these stress directions show great variability, even over short distances. Therefore statistical methods are helpful in deciding which apparent variations in space or in time are significant. First, the interpolation technique of Bird and Li [1996] is used to interpolate stress directions to a grid of evenly spaced points in each of seventeen 5-m.y. time steps since 85 Ma. Then, a t test is used to search for stress direction changes between pairs of time windows whose sense can be determined with some minimum confidence. Available data cannot resolve local stress provinces, and only the broadest changes affecting country-sized regions are reasonably certain. During 85-50 Ma, the most compressive horizontal stress azimuth (sigma) over cap (1H) was fairly constant at similar to68degrees (United States) to 75degrees (Mexico). During 50-35 Ma, both counterclockwise stress changes (in the Pacific Northwest) and clockwise stress changes (from Nevada to New Mexico) are seen, but only locally and with about 50% confidence. A major stress azimuth change by similar to90degrees occurred at 33 +/- 2 Ma in Mexico and at 30 +/- 2 Ma in the western United States. This was probably an interchange between (sigma) over cap (1) and (sigma) over cap (3) caused by a decrease in horizontal compression and/or an increase in vertical compression. The most likely cause was the rollback of horizontally subducting Farallon slab from under the southwestern United States and northwest Mexico, which was rapid during 35-25 Ma. After this transition, a clockwise rotation of principal stress axes by 36degrees-48degrees occurred more gradually since 22 Ma, affecting the region between latitudes 28degreesN and 41degreesN. This occurred as the lengthening Pacific/North America transform boundary gradually added dextral shear on northwest striking planes to the previous stress field of SW-NE extension.